Low cure cathodic electrodeposition coatings

ABSTRACT

A low temperature cure electrodepositable cathodic resin is disclosed. The resin is the reaction product of a polyepoxide amine adduct and a novel blocked crosslinking agent. The crosslinking agent of our invention is tetramethylxylene diisocyanate blocked with any suitable blocking agent. The resulting resin can be cured at relatively low temperatures (250°-275° F.), which potentially allows the use of the resin with articles containing plastics.

TECHNICAL FIELD

The field of art to which this invention pertains is electrodepositableepoxy resins containing crosslinking agents to be used in cathodicelectrocoat processes.

BACKGROUND ART

The coating of electrically conductive substrates by electrodepositionis a well known and important industrial process. (For instance,electro-deposition is widely used in the automotive industry to applyprimers to automotive substrates). In this process, a conductive articleis immersed as one electrode in a coating composition made from anaqueous emulsion of film-forming polymer. An electric current is passedbetween the article and a counter-electrode in electrical contact withthe aqueous emulsion, until a desired coating is produced on thearticle. The article to be coated is the cathode in the electricalcircuit with the counter-electrode being the anode.

Resin compositions used in cathodic electro-deposition baths are alsowell known in the art. These resins are typically manufactured frompolyepoxide resins which have been chain extended and adducted toinclude a nitrogen. The nitrogen is typically introduced throughreaction with an amine compound. Typically these resins are blended witha crosslinking agent and then salted with an acid to form a wateremulsion which is usually referred to as a principal emulsion.

The principal emulsion is combined with a pigment paste, coalescentsolvents, water, and other additives at the coating site to form theelectro-deposition bath. The electro-deposition bath is placed in aninsulated tank containing the anode. The article to be coated is madethe cathode and is passed through the tank containing theelectro-deposition bath. The thickness of the coating is a function ofthe bath characteristics, the electrical operating characteristics, theimmersion time, and so forth.

The coated object is removed from the bath after a set amount of time.The object is rinsed with deionized water and the coating is curedtypically in an oven at sufficient temperature to produce crosslinking.

The prior art of cathodic electrodepositable resin compositions, coatingbaths, and cathodic electro-deposition processes are disclosed in U.S.Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307.

All current commercial cathodic electrocoat processes must be cured at ahigh temperature (e.g., 325° F. to 360° F.). However, at these high baketemperatures it is not possible to coat many plastic substrates becausethey ten to distort at high temperature. Nevertheless, there is a pushwithin the automotive industry to go toward plastic substrates forcertain uses. Thus there is a need for an electrocoat process that willallow curing at lower temperatures (e.g., 250° F.-275° F.) so that thebaking process will not distort the plastic substrate.

The push toward low cure electrocoat systems has thus for beenfrustrated by bath instability, film roughness, poor coating corrosionresistance and poor chip resistance. These characteristics areinterrelated and are thought to be at least partially caused bypremature curing of the film while still in the electrocoat bath.

It is very important that the electrodeposited layer be of high qualityeven though it typically will be covered with top coats. Defects in theelectrodeposited layer such as cratering or roughness may be evidentthrough the top coats.

In order that plastic substrates may be used in electrocoat processesthere is a need for an electrocoat system that will allow for low curebut also have a stable bath, smooth film, good corrosion resistance andacceptable chipping characteristics.

SUMMARY OF THE INVENTION

It has been discovered that by using a novel crosslinking agent,principal emulsions can be prepared that result in electro-depositionbaths that are stable, will cure at lower temperatures and form filmsthat are smooth and coatings with good corrosion and chip resistance.More specifically, a cathodic electrodepositable resin composition ofthe type comprising an epoxy amine adduct, blended with a blockedtetramethylxylene diisocyanate (TMXDI) crosslinker, and then salted toform a principal emulsion is disclosed. The improvement therein beingthe use of blocked tetramethylxylene diisocyanate as the crosslinkingagent and the use of a secondary amine with a primary hydroxyl group toform the epoxy amine adduct.

The resulting coating composition cures at 250°-275° F. and has a pH ofabout 6.0 with low conductivity. The coating has a smooth appearancewith good corrosion resistance, high rupture voltage and good throwpower.

DETAILED DESCRIPTION OF THE INVENTION

As previously mentioned, it is well known that most principal emulsionsin electro-deposition baths comprise an epoxy amine adduct blended witha cross-linking agent and salted with an acid in order to get a watersoluble product. Typical crosslinkers used in the prior art arealiphatic and aromatic isocyanates such as hexamethylene diisocyanate,toluene diisocyanate, methylene diphenyl diisocyanate and so forth.These isocyanates are pre-reacted with a blocking agent such as methylethyl ketoxime which blocks the isocyanate functionality (i.e. thecrosslinking functionality). Upon heating the blocking agents separateand crosslinking occurs.

The key to choosing a cross-linking agent which is suitable for use atlow cure conditions is finding one with the right reactivity and thecorrect unblocking temperature.

The cross-linking agent of our novel process is tetramethylxylenediisocyanate. TMXDI is first reacted with a polyol such as trimethylolpropane (TMP) or other polyol containing two or more hydroxy functionalgroups. In our preferred mode the polyol is trimethylolpropane. Theratio of TMXDI to TMP is about 3:1. The resulting polyisocyanate is thenreacted with a blocking agent under reaction conditions well known inthe art until no free isocyanates are present. U.S. Pat. Nos. 4,031,050and 3,947,358 show these reaction conditions. The blocking agents usefulin this invention include acetone oxime, methyl amyl ketoxime, andparticularly preferred is methyl ethyl ketoxime. The blocking agent isusually added in an equivalent ratio of about 1:1 to the polyisocyanate.In addition, the reactor should also be charged with an organic solventsuch as methyl ethyl ketone, methyl isobutyl ketone, and so forth.

In our preferred mode, the methyl ethyl ketoxime and TMXDI-TMP adductare reacted at 50° C. to 100° C. for about one hour.

The polyepoxide resins which are used in the practice of the inventionare polymers having a 1,2-epoxy equivalency greater than one andpreferably about two, that is, polyepoxides which have on an averagebasis two epoxy groups per molecule. The preferred polyepoxides arepolyglycidyl ethers of cyclic polyols. Particularly preferred arepolyglycidyl ethers of polyhydric phenols such as bisphenol A. Thesepolyepoxides can be produced by etherification of polyhydric phenolswith epihalohydrin or dihalohydrin such as epichlorohydrin ordichlorohydrin in the presence of alkali. Examples of polyhydric phenolsare 2,2-bis-(4-hydroxy-3-tertiarybutylphenyl)propane,1,1-bis-(4-hydroxyphenyl)ethane, 2-methyl-1,1-bis-(4-hydroxyphenyl)propane, 2,2-bis-(4-hydroxy-3-tertiarybutylphenyl)propane,bis-(2-hydroxynaphthyl methane, 1,5-dihydroxy-3-naphthalene or the like.

Besides polyhydric phenols, other cyclic polyols can be used inpreparing the polyglycidyl ethers of cyclic polyol derivatives. Examplesof other cyclic polyols would be alicyclic polyols, particularlycycloaliphatic polyols, such as 1,2-cyclohexanediol,1,4-cyclohexanediol, 1,2-bis(hydroxymethyl)cyclohexane,1,3-bis-(hydroxymethyl)cyclohexane and hydrogenated bisphenol A.

The polyepoxides have molecular weights of at least 200 and preferablywithin the range of 200 to 2000, and more preferably about 340 to 2000.

The polyepoxides are preferably chain extended with a polyether or apolyester polyol which increases rupture voltage of the composition andenhances flow and coalescence. Examples of polyether polyols andconditions for chain extension are disclosed in U.S. Pat. No. 4,468,307.Examples of polyester polyols for chain extension are disclosed in U.S.Pat. No. 4,148,772.

The polyepoxide is reacted with a cationic group former, for example, anamine and then salted with an acid.

The amines used to adduct the epoxy resin are monoamines, particularlysecondary amines with primary hydroxyl groups. When you react thesecondary amine containing the primary hydroxyl group with the terminalepoxide groups in the polyepoxide the result is the amine/epoxy adductin which the amine has become tertiary and contains a primary hydroxylgroup. Typical amines that can be used in the invention are methylethanol amine, diethanolamine, and so forth. Our preferred amine isdiethanol amine.

Mixtures of the various amines described above can be used. The reactionof the secondary amine with the polyepoxide resin takes place uponmixing the amine with the polyepoxide. The reaction can be conductedneat, or, optionally in the presence of suitable solvent. The reactionmay be exothermic and cooling may be desired. However, heating to amoderate temperature, that is, within the range of 50° to 150° C., maybe used to hasten the reaction.

The reaction product of secondary amine with the polyepoxide resinattains its cationic character by at least partial neutralization withacid. Examples of suitable acids include organic and inorganic acidssuch as formic acid, acetic acid, lactic acid, and phosphoric acid. Theextent of neutralization will depend upon the particular productinvolved. It is only necessary that sufficient acid be used to dispersethe product in water. Typically, the amount of acid used will besufficient to provide at least 30 percent of the total theoreticalneutralization. Excess acid beyond that required for 100 percent totaltheoretical neutralization can also be used.

The extent of cationic group formation of the resin should be selectedsuch that when the resin is mixed with aqueous medium, a stabledispersion will form. A stable dispersion is one which does not settleor is one which is easily redispersible if some sedimentation occurs. Inaddition, the resin should be of sufficient cationic character that thedispersed resin particles will migrate towards the cathode when there isan electrical potential between an anode and a cathode immersed in theaqueous dispersion.

In general, most of the cationic resins prepared by the process of theinvention contain from about 40 to 80, preferably from about 50 to 70milliequivalents of cationic group per hundred grams of resin solids.

The cationic resinous binder should preferably have average molecularweights, as determined by gel permeation chromatography using apolystyrene standard, of less than 10,000, more preferably less than5,000 and most preferably less than 3,000 in order to achieve highflowability.

The cationic resin and the blocked isocyanate are the principal resinousingredients in the electrocoating composition and are usually present inamounts of about 30 to 50 percent by weight of solids.

Besides the resinous ingredients described above, the electrocoatingcompositions usually contain a pigment which is incorporated into thecomposition in the form of a paste. The pigment paste is prepared bygrinding or dispersing a pigment into a grinding vehicle and optionalingredients such as wetting agents, surfactants, and defoamers. Pigmentgrinding vehicles are well known in the art. After grinding, theparticle size of the pigment should be as small as practical, generally,a Hegman grinding gauge of about 6 to 8 is usually employed.

Pigments which can be employed in the practice of the invention includetitanium dioxide, basic lead silicate, strontium chromate, carbon black,iron oxide, clay and so forth. Pigments with high surface areas and oilabsorbencies should be used judiciously because they can have anundesirable effect on coalescence and flow.

The pigment-to-resin weight ratio is also fairly important and should bepreferably less than 0.5:1, more preferably less than 0.4:1, and usuallyabout 0.2 to 0.41. Higher pigment-to-resin solids weight ratios havealso been found to adversely affect coalescence and flow.

The coating compositions of the invention can contain optionalingredients such as wetting agents, surfactants, defoamers and so forth.Examples of surfactants and wetting agents include alkyl imidazolinessuch as those available from Ciba-Geigy Industrial Chemicals as "AmineC", acetylenic alcohols available from Air Products and Chemicals as"Surfynol 104". These optional ingredients, when present, constitutefrom about 0 to 20 percent by weight of resin solids. Plasticizers areoptional ingredients because they promote flow. Examples are highboiling water immiscible materials such as ethylene or propylene oxideadducts of nonyl phenols or bisphenol A. Plasticizers are usually usedat levels of about 0 to 15 percent by weight resin solids.

Curing catalysts such as tin catalysts are usually present in thecomposition. Examples are dibutyltin dilaurate and dibutyltin oxide.When used, they are typically present in amounts of about 0.05 to 1percent by weight tin based on weight of total resin solids.

The electrodepositable coating compositions of the present invention aredispersed in aqueous medium. The term "dispersion" as used within thecontext of the present invention is believed to be a two-phasetranslucent or opaque aqueous resinous system in which the resin is inthe dispersed phase and water the continuous phase. The average particlesize diameter of the resinous phase is about 0.1 to 10 microns,preferably less than 5 microns. The concentration of the resinousproducts in the aqueous medium is, in general, not critical, butordinarily the major portion of the aqueous dispersion is water. Theaqueous dispersion usually contains from about 3 to 50 percentpreferably 5 to 40 percent by weight resin solids. Aqueous resinconcentrates which are to be further diluted with water, generally rangefrom 10 to 30 percent by total weight solids.

Besides water, the aqueous medium may also contain a coalescing solvent.Useful coalescing solvents include hydrocarbons, alcohols, esters,ethers and ketones. The preferred coalescing solvents include alcohols,polyols and ketones. Specific coalescing solvents include monobutyl andmonohexyl ethers of ethylene glycol, and phenyl ether of propyleneglycol. The amount of coalescing solvent is not unduly critical and isgenerally between about 0 to 15 percent by weight, preferably about 0.5to 5 percent by weight based on total weight of the resin solids.

EXAMPLE A

Backbone Resin

The following components were charged into a suitable reactor vessel:1217 parts "Epon 828" (a diglycidyl ether of bisphenol A from ShellChemical Co.) having an epoxy equivalent weight of 188; 444 parts ofethoxylated Bisphenol A having an hydroxy equivalent weight of 227("Synfac 8009" from Milliken Co.); 355 parts of Bisphenol A; and 106parts of xylene. The charge was heated to 145° C. under a dry nitrogenblanket and 2.1 parts of benzyl dimethyl amine. The reaction mixture wasfurther heated to 160° C. and held for 1 hour. An additional 4.2 partsof benzyl dimethyl amine were added, and the mixture held at 147° C.until the desired weight per epoxide (WPE) was achieved. The mixture wascooled to 98° C., and 203 parts of diethanolamine were added. Themixture was held at 120° C. for 1 hour, and then 600 parts of methylisobutyl ketone were added.

EXAMPLE B

Crosslinker

A methyl ethyl ketoxime polyisocyanate was prepared by slowly charging182 parts of methyl ethyl ketoxime in a reaction vessel containing 955parts of "Cythane 3160" (the adduct of meta tetramethyl xylenediisocyanate and trimethyolpropane, 78.8% non-volatiles and 359isocyanate equivalent weight from American Cyanamide) and 198 parts ofmethyl isobutyl ketone at 65° C. under a nitrogen blanket. The mixturewas maintained at 85° C. for one hour until essentially all theisocyanate was consumed as indicated by infrared scan.

    ______________________________________                                        Example C                                                                                          Wt.   NV                                                 ______________________________________                                        QUATERNIZING AGENT                                                            2-Ethylhexanol half    320.0   304                                            capped TDI in MIBK                                                            Dimethylethanolamine   87.2    87.2                                           Aqueous Lactic Acid Solution                                                                         ll7.6   88.2                                           2-Butoxyethanol        39.2                                                   PIGMENT GRINDING VEHICLE                                                      "Epon 829"             720     682                                            Bisphenol A            289.6   289.6                                          2-Ethylhexanol half    406.4   386.1                                          capped TDI in MIBK                                                            Quaternizing Agent (from above)                                                                      496.3   421.9                                          Deionized Water        71.2                                                   2-Butoxyethanol        149.0                                                  ______________________________________                                    

The quaternizing agent was prepared by adding dimethylethanolamine tothe ethylhexanol half-capped Toluene diisocyanate in a suitable reactionvessel at room temperature. The mixture exothermed and was stirred forone hour at 80° C. Lactic acid was then charged followed by the additionof 2-butoxyethanol. The reaction mixture was stirred for about one hourat 65° C. to form the desired quaternizing agent.

To form the pigment grinding vehicle "Epon 829" (a diglycidyl ether ofbisphenol A from Shell Chemical Co.) and Bisphenol A were charged undera nitrogen atmosphere to a suitable reaction vessel and heated to150°-160° C. to initiate an exothermic reaction. The reaction mixturewas permitted to exotherm for one hour at 150°-160° C. The reactionmixture was then cooled to 120° C. and the 2-ethylhexanol half-cappedtoluene diisocyanate was added. The temperature of the reaction mixturewas held at 110°-120° C. for one hour, followed by the addition of the2-butoxyethanol. The reaction mixture was then cooled to 85°-90° C.,homogenized and then charged with water, followed by the addition of thequaternizing agent (prepared above). The temperature of the reactionmixture was held at 80°-85° C. until an acid value of about 1 wasobtained. The reaction mixture had a solids content of 55 percent.

The above ingredients were mixed together and ground in a mill to aHegman No. 7 grind.

EXAMPLE 1

Pigment

A pigment dispersion was made by grinding 141.9 parts of the grind resin(from Example C ), 240 parts deionized water, 217.8 parts of aluminumsilicate, 37.3 parts of lead silicate, 6.2 parts of carbon black, 6.2parts of basic lead silica chromate, 87.1 parts of titanium dioxide,13.7 parts dibutyl tin oxide in a vertical sandgrinder until the maximumparticle size of the mixture was about 12 microns. Then, 238.4 parts ofwater were added.

EXAMPLE 2

Principal emulsion

The following materials were charged into a suitable reaction vessel:187.6 parts backbone resin Example A, 143.2 parts crosslinker Example B,6.1 parts Glacial acetic acid, 1.8 parts surfactant solution [a mixtureof "Surfynol 104" (from Air Products), "Amine C" (from Ciba-Geigy), andbutyl cellosolve in equal portions, plus a small amount of acetic acid],36.3 parts PPH (propylene glycol phenyl ether) and allowed to mix untila homogenous solution was achieved. 325.5 parts of deionized water wereadded under high shear to form the emulsion. The solvents contained inthe backbone and crosslinker were allowed to evaporate from the emulsionat room temperature under slow agitation.

EXAMPLE 3

Low bake cathodic electrocoat system

A cationic electrodepositable paint was prepared by blending 651.0 partsof the principal emulsion Example 2, 168.7 parts pigment dispersionExample 1, and 680.0 parts deionized water. The bath paint had a totalsolids content of 19.5 wt. %, pH 7.1 and conductivity 1856 microSiemens. A phosphated steel panel electrocoated at 200 Volts gave asmooth film of 23 microns thickness and excellent cure at 121° C. (250°F.) ten minutes metal temperature. This material has good corrosionresistance over phosphated cold roll steel and pre-treated metals(galvanized), as indicated by the results from a cyclic corrosion test.

We claim:
 1. A resin composition for use as the film forming componentin a cathodic electro-deposition process, comprising the reactionproduct of (A) a polyepoxide amine adduct and (B) a trimerizedcross-linking agent, said crosslinking agent being formed by bringinginto contact about three moles of tetramethyl xylene diisocyanate andabout one mole of trimethylolpropane under conditions that will cause areaction and then blocking the reaction product with about three molesof an oxime, wherein the resin composition can be cured at temperaturesbelow 275° C.
 2. The resin composition of claim 1 in which saidpolyepoxide amine adduct is formed by mixing a polyepoxide with asecondary amine having primary hydroxyl groups at appropriate reactionconditions.
 3. The resin composition of claim 2 in which said secondaryamine having primary hydroxyl groups is selected from the groupconsisting of methyl ethanol amine or diethanol amine.
 4. The resincomposition of claim 2 in which said polyepoxide are polymers having anaverage of about two epoxy groups per molecule.
 5. The resin compositionof claim 4 in which said polyepoxides are polyglycidyl ethers ofpolyhydric phenols.
 6. The resin composition of claim 1 in which saidblocking agent is selected from the group consisting of acetone oxime,methyl ethyl ketoxime, methyl amyl ketoxime, or methyl isobutylketoxime.
 7. A method of coating an electrically conductive article withthe film forming resin of claim 1 using cathodic electro-depositioncomprising:(1) forming a polyepoxide amine adduct; (2) mixing saidpolyepoxide amine adduct with a crosslinker formed fromtetramethylexylene diisocyanate blocked with an oxime; (3) adding acidand water to the mixture of the epoxy amine adduct and the crosslinkerthereby forming a principal emulsion; (4) mixing the principal emulsionwith the appropriate amount of water and pigment paste to form anelectrocoat bath; (5) immersing the article in the electrocoat bath; and(6) passing a direct current across the article.